Thin film transistor manufacturing method and thin film transistor

ABSTRACT

A first insulation film is formed as a gate insulation film of a thin film transistor, and a gate electrode is formed on the gate insulation film. Then, dopant is implanted to form source and drain regions. A second insulation film having refractive index n1 and film thickness d2 is formed to cover the first insulation film and gate electrode as an interlayer insulation film. After forming the second insulation film, laser with wavelength λ is applied to activate the dopant. The film thicknesses d1 and d2 of the first and second insulation films satisfy conditions against the laser wavelength λ for forming a reflection protective film at regions where activation is necessary. At the same time, the film thicknesses d1 and d2 are set in a way that the interlayer insulation film on the gate electrode forms a reflective film. This reduces the thermal damage to the gate electrode from the laser during dopant activation.

FIELD OF THE INVENTION

The present invention relates to the field of methods for manufacturingplycrystal silicon thin film transistors and thin film transistors asemployed in liquid crystal display devices and input and output devicesincluding image sensors.

BACKGROUND OF THE INVENTION

The electron mobility of a polycrystal silicon thin film transistor isgreater by a factor of 100 than that of an amorphous silicon thin filmtransistor. The use of polycrystal silicon thin film transistors allowsthe miniaturization of elements and the denser mounting of drivingcircuits on one substrate. In the field of liquid crystal displaydevices, polycrystal silicon thin film transistors are recently used inthin film transistor arrays with built-in driving circuits. These thinfilm transistor arrays with built-in driving circuits have been madepossible by the development of technology to manufacture arrays on glasssubstrates which can be easily enlarged.

To form polycrystal thin film transistors at low temperatures, thedevelopment of a method for activating the dopant implanted into thepolycrystal silicon thin film at low temperatures is important as wellas technology to form polycrystal silicon thin film at low temperatures.Low temperature crystallization using excimer laser annealing is oftenemployed to form good polycrystal silicon thin films on large substratesat low temperatures.

For example, IEEE Electron Device Letters, Vol. EDL-7, No. 5, May 1986,pp. 276-278, discloses technology related to excimer laser annealing. Ingeneral, thermal annealing is used for activation, but the activationrate significantly drops as a result of reducing the processingtemperature.

Rapid thermal annealing (RTA) and excimer laser activation are proposedas methods for improving the dopant activation rate at low temperaturesto counteract the above disadvantage. SID97 M/52: Recent Advances inRapid Thermal Processing of Polysilicon TFT LCDs discloses RTAactivation, and the Extended Abstract of the 18th (1986) InternationalConference on Solid State Devices and Materials, pp. 225-228, disclosesexcimer laser activation.

FIGS. 3A to 3D show process flow charts describing a conventional methodof manufacturing polysilicon thin film transistors for the active matrixarrays used in liquid crystal display devices. As shown in FIG. 3A, asilicon oxide film which becomes a buffer layer 12 is formed on atransparent glass substrate 11 using the plasma CVD method. Amorphoussilicon (a-Si) film is then deposited using the plasma CVD methodwithout exposing the substrate 11, on which the buffer layer 12 isformed, to air.

Next, a thermal treatment is applied to reduce the hydrogen in the a-Sifilm. The a-Si film is polycrystallized by excimer laser annealing toform a poly-Si film 13 a. Finally, the poly-Si film 13 a is processedinto the size and shape required for a TFT.

Next, a silicon oxide film which becomes a gate insulation film 14 isformed. A gate electrode 15 typically made of Al alloy is formed anddopant is implanted to form a Lightly Doped Drain (LDD) region 13 b inthe thin film transistor as shown by an arrow 100 in FIG. 3A. As shownin FIG. 3B, a mask for implanting dopant into the source and drainregions is then formed using a photo resist 25 in a manner to cover theLDD region 13 b of the thin film transistor. A large quantity ofphosphorus ion, the dopant, is implanted into the source region 21 anddrain region 22 by ion implantation, as shown by an arrow 100 in FIG.3B. The source region 21 and drain region 22 which have highconcentrations of dopant are called a SD region 13C.

Since the implanted dopant is electrically inactive, excimer laser lightis applied, as shown by an arrow 101 in FIG. 3C, to activate it.

Then, as shown in FIG. 3D, a silicon oxide film which becomes aninterlayer insulation film 16 is formed and contact holes 17 a and 17 bare opened on the insulation film in the source region 21 and drainregion 22. A layered film of Ti and Al is formed and processed to formSD wirings 18 a and 18 b.

Finally, a protective insulation film 23 made of silicon nitride isformed, and annealed in a hydrogen atmosphere. Hydrogen annealing fillsthe empty ionic bonds in the polycrystal silicon thin film withhydrogen, enabling the characteristics of the thin film transistor to beimproved.

However, the conventional method of activation using an excimer lasercauses a high degree of thermal damage to the gate electrode 15. Morespecifically, as shown in FIG. 3C, an irradiated excimer laser light isapplied to and absorbed by the polycrystal silicon through the gateinsulation film 14 at the source region 21 and drain region 22 of thethin film transistor. The laser light applied to the gate electrode 15region is also directly absorbed by the gate metal, causing the gateelectrode's temperature to rise. If metals with high melting points suchas W, Mo, and Cr are used for the gate electrode 15, cracks or peelingof the gate electrode 15 may occur as a result of thermal shock due tolaser irradiation. If Al alloy is used for the gate electrode 15,quality problems such as an increase in hillocks may occur. Hillocks arethe phenomenon whereby the material surface becomes bumpy as a result oftemperature rise.

The present invention provides a thin film transistor manufacturingmethod and thin film transistor which reduces the thermal damage to gateelectrodes caused by laser irradiation during the manufacture of thinfilm transistors which includes the process of dopant activation bylaser irradiation.

SUMMARY OF THE INVENTION

A method for manufacturing thin film transistors in accordance with anexemplary embodiment of the present invention includes the steps offorming a semiconductor thin film on a transparent substrate; forming afirst insulation film having a refractive index n1 and film thickness d1on the semiconductor thin film as a gate insulation film; forming a gateelectrode on the first insulation film; implanting dopant into thesemiconductor thin film; forming a second insulation film havingrefractive index n2 and film thickness d2 in a way to cover the firstinsulation film and gate electrode; and activating dopant implanted byapplying laser with wavelength λ after forming the second insulationfilm. In this configuration, the film thicknesses d1 and d2 practicallysatisfy a set of Formulae (1) and (2) as follows:

d2*n2=2*m*λ/4  (1)

d1*n1+d2*n2=(2*m1−1)*λ/4  (2)

Here, m and m1 are any given positive integer.

These film thicknesses enable the laser light to be reflected off thegate electrode and absorbed at portions other than the gate electrode.This allows a reduction in the thermal damage to the gate electrode bylaser irradiation, and also achieves efficient dopant activation by thelaser.

Another exemplary embodiment of the present invention refers to a methodfor manufacturing thin film transistors including the steps of formingthe semiconductor film on the transparent substrate; forming the firstinsulation film having refractive index n1 and film thickness d1 on thesemiconductor thin film as a gate insulation film; forming the gateelectrode on the first insulation film; implanting dopant into thesemiconductor thin film after forming the gate electrode; forming thesecond insulation film having refractive index n2 and film thickness d2in a way to cover the first insulation film and gate electrode; andactivating dopant implanted by laser irradiating with a wavelength λafter forming the second insulation film.

In this configuration, the film thickness d1 of the first insulationfilm and film thickness d2 of the second insulation film fall in a rangepractically satisfying a set of Formulae (5) and (6) when m and m1 areany given positive integers.

abs{d2*n2−2*m*λ/4}<λ8  (5);

and

abs{(d2*n2+d1*n1)−(2*m1−1)*λ/4}<λ/8  (6);

The above acceptable range for the film thicknesses d1 and d2 allows toreduce the thermal damage to the gate electrode by laser irradiation,and also to achieve efficient dopant activation by the laser.

In these methods for manufacturing thin film transistors, the firstinsulation film is silicon oxide made by decomposing a gaseous materialcontaining at least organic silicon material by plasma.

A thin film transistor of the present invention includes a semiconductorthin film formed on a transparent substrate; a first insulation filmhaving refractive index n1 and film thickness d1 formed on thesemiconductor thin film as a gate insulation film; a gate electrodeformed on the first insulation film; dopant implanted into thesemiconductor thin film; and a second insulation film having refractiveindex n2 and film thickness d2 formed in a way to cover the firstinsulation film and gate electrode. Implanted dopant is activated byapplying the laser with wavelength λ. In this configuration, the filmthicknesses d1 and d2 practically satisfy a set of Formulae (1) and (2)when m and m1 are any given positive integers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1D are sectional views illustrating processes of a methodfor manufacturing thin film transistors in a preferred embodiment of thepresent invention.

FIG. 2A shows a characteristics chart illustrating the relation betweenthe film thickness of an insulation film and reflectance of laser light.

FIG. 2B is a sectional view of regions A and B in the thin filmtransistor.

FIGS. 3A to 3D are sectional views illustrating processes of aconventional method for manufacturing thin film transistors.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A method for manufacturing thin film transistors in a preferredembodiment of the present invention is described below with reference toFIGS. 1A to 1D. As shown in FIG. 1A, a silicon oxide film of 400 nmthick is formed on a transparent glass substrate 11 using the plasma CVDmethod to form a buffer layer 12. Then, amorphous silicon (a-Si) isdeposited up to 50 nm thick using the plasma CVD method without exposingthe glass substrate 11, on which the silicon oxide thin film is formed,to air. To reduce hydrogen in the a-Si film, the glass substrate 11 isthermally treated at 450° C. for 90 minutes under the reduced nitrogenatmosphere of 1 Torr. The a-Si film is polycrystalized by excimer laserannealing to form a poly-Si film 13 a which is a non-single crystalsemiconductor thin film. As for excimer laser, XeCl excimer laser with awavelength of 308 nm is used, and irradiated in a vacuum. Its energydensity is 350 mJ/cm², and average irradiation shots are 35 shots/point.

After crystallizing the a-Si film to form the poly-Si film 13 a, thepoly-Si film 13 a is processed into the size and shape required for aTFT. A silicon oxide film of 50 nm thick is deposited to form a gateinsulation film 14 which is a first insulation film. This silicon oxidefilm is made from a mixed gas of oxygen gas and TEOS(tetraethylorthosilicate, Si(OCH₂CH₃)₄) gas, which is an organic siliconmaterial using the plasma CVD method. The film thickness is set to 45nm.

A gate electrode 15 made of Al alloy is then formed. An LDD region 13 bof the thin film transistor is formed by implanting dopant using thegate electrode 15 as a mask. Phosphorus ion is excited at theaccelerated voltage of 70 keV, and implanted to the direction of anarrow 100 for a dose rate of 10¹³/cm². After forming the LDD region 13b, photo resist 25 is applied to cover the LDD region 13 b of the thinfilm transistor, as shown in FIG. 1B, to form a mask for implantingdopant into the source region 21 and drain region 22. The LDD region isnot essential, but is effective for reducing the OFF-state current ofthe thin film transistor.

After implanting the dopant, a silicon oxide film of 215 nm thick isformed as a second insulation film, as shown in FIG. 1C, to form aninterlayer insulation film 16. Then, a short-wave excimer laser isapplied to activate implanted dopant as shown by an arrow 101 in FIG.1C. The laser used for activation is XeCl excimer laser, and has awavelength λ of 308 nm, and a half width of 30 nm. Its energy density is300 mJ/cm², and the average irradiation shots are 20 shots/point.

The gate insulation film 14, which is the first insulation film, has thefilm thickness d1=50 nm and refractive index n1=1.46. The interlayerinsulation film 16, which is the second insulation film, formed on thegate insulation film 14 has the film thickness d2=215 nm and refractiveindex n1=1.46. Accordingly, insulation film on the source region 21 anddrain region 22 of the thin film transistor which requires dopantactivation has a film thickness of d1+d2=265 nm. The refractive index n1is both 1.46.

FIG. 2A shows the reflectance of laser light against the thickness ofinsulation film when the laser light enters the insulation film (SiO₂)from the air. FIG. 2B shows a sectional view of the thin film transistorduring activation by the laser light. This figure corresponds to thesectional view in FIG. 1C. The reflectance of the laser light enteringthe insulation film from the air repeats the maximum and minimumreflectance in a cycle of λ/(4*n), as shown in FIG. 2A, when the laserwavelength is λ and refractive index of the insulation film is n.

FIG. 2A shows the case when the wavelength λ of excimer laser is 308 nm,and refractive index n of the insulation film (SiO₂) is 1.46. In thiscase, a half cycle λ/(4*n) of the reflectance is 52.7 nm. Accordingly,since laser light enters the interlayer insulation film (d2=215 nm) onthe region A shown in FIG. 2B, which is the gate electrode, thereflectance becomes almost the maximum as shown in FIG. 2A, and incidentlaser energy is scarcely absorbed by the gate electrode 15 of the thinfilm transistor

On the other hand, on the region B shown in FIG. 2B, which is the sourceregion 21, drain region 22, and LDD region 13 b of the thin filmtransistor, the laser light enters the film having the thickness of thesum of the film thickness d1=50 nm of the gate insulation film and thefilm thickness d2=215 nm of the interlayer insulation film i.e.,d1+d2=265 nm. Accordingly, the reflectance is almost minimum as shown inFIG. 2A, and incident laser energy reaches the bottom poly-Si film 13 amost efficiently. The poly-Si film 13 a is thus most efficientlyannealed, and dopant implanted is satisfactorily activated.

The above findings may take the next numerical forms.

Ideal conditions are achieved when the interlayer insulation filmthickness d2 is an even multiple of a half period of the reflectance,i.e.,

d2=2*m*λ/(4*n1); and

the sum d1+d2 of both insulation film thicknesses is an odd multiple ofa half period of the reflectance, i.e.,

d1+d2=(2*m1−1)*λ/(4*n1).

Here m and m1 are any given positive integers.

These formulae may then be rearranged as follows:

d2*n1=2*m*λ/4; and

(d1+d2)*n1=(2*m1−1)*λ/4.

These formulae may be generalized for the case when refractive index n2of the interlayer insulation film is different from refractive index n1of the gate insulation film as follows:

d2*n2=2*m*λ/4  (1);

and

d2*n2+d1*n1=(2*m1−1)*λ/4  (2).

In other words, the dopant is ideally activated by the laser when a setof Formulae (1) and (2) are satisfied.

After dopant activation, contact holes 17 a and 17 b are opened on theinterlayer insulation film 16 as shown in FIG. ID, and then SD wiring 18a and 18 b made of a Ti and Al layered film are respectively formed.Lastly, a protective insulation film 23 made of silicon nitride isformed, and annealed in a hydrogen atmosphere. Accordingly, empty ionicbonds in the polycrystal silicon thin film are filled with hydrogen toimprove characteristics of the thin film transistor.

Annealing in the above description is preferably conducted at between250° C. and 400° C. for 30 minutes to 3 hours. Here, annealingtemperature is 350° C.; and annealing time is 1 hour. A thin filmtransistor manufactured using the manufacturing method of the presentinvention demonstrates mobility of 150 cm²/V•sec and Vth=2.0 V. Anincrease in hillocks is not observed in a process of dopant activationby the laser even if Al alloy is used for the gate electrode 15.

The manufacturing method of the present invention thus enables excimerlaser light to be reflected off the interlayer insulation film 16 on thegate electrode 15. On the other hand, interlayer insulation film 16 andgate insulation film 14 on the source region 21, drain region 22, andLDD region 13 b of the thin film transistor prevents reflection of theexcimer laser. This allows efficient absorption of laser energy atregions requiring dopant activation, and at the same time, preventsabsorption of laser energy at the gate electrode which requires to avoidtemperature rise. Accordingly, materials which likely to cause hillocks,cracks and the like by temperature rise, such as Al and metals having alarge stress and high melting point including Cr, Mo, W, and Ni, may beused for gate wiring.

As shown in FIG. 2A, minimum and maximum reflectance repeat in everyinsulation film thickness of λ/(4*n1) [nm] against wavelength λ of thelaser in use and refractive index n1 of the insulation film.Accordingly, errors in the film thickness of the insulation film arepreferably within the range of a half of a minimum interval where thereflectance becomes the maximum and minimum, i.e,±λ/(8*n1) [nm]. If thiscondition is quantified, the film thickness d1 of the gate insulationfilm and the film thickness d2 of the interlayer insulation film maysatisfy a set of the following Formulae (3) and (4) when m and m1 areany given positive integers:

abs{d2*n1−2*m*λ/4}<λ/8  (3);

and

abs{(d2+d1)*n1−(2*m1−1)*λ/4}<λ/8  (4).

The preferred embodiment uses the same material for the gate insulationfilm and interlayer insulation film, which means the same refractiveindex n1 for both films. However, there is no need to use materialshaving the same refractive index. When materials having differentrefractive index are used for the gate insulation film and interlayerinsulation film, the film thicknesses d1 and d2 may satisfy a set of thefollowing Formulae (5) and (6) when m and m1 are any given positiveintegers:

abs{d2*n2−2*m*λ/4}<λ/8  (5);

and

abs{(d2*n2+d1*n1)−(2*m1−1)*λ/4}<λ/8  (6);

where film thickness of the gate insulation film is d1 and itsrefractive index is n1, and film thickness of the interlayer insulationfilm is d2 and its refractive index is n2.

The same effects as described in the preferred embodiment are achievablewhen the above Formulae (5) and (6) are satisfied.

The use of organic silicon material, such as TEOS gas decomposed byplasma for making the gate insulation film, as described in thepreferred embodiment, is effective for improving the reliability of thethin film transistor because a damage to the base layer at depositingthe film is little.

As described above, the present invention enables formation of anoptical reflective film on the gate electrode against the laser light,and formation of a reflection preventive film on the source and drainregions of the thin film transistor when the laser light is applied toactivate the dopant. This enables the gate electrode to reflect thelaser beam during activation, and at the same time, allows the regionswhere dopant is implanted to absorb energy efficiently. Accordingly,cracks and peeling of the gate electrode is preventable even in laserannealing conditions achieving sufficient activation rate. As a result,the present invention significantly improves the yield in themanufacturing of thin film transistors.

Furthermore, the use of silicon oxide film made by decomposing anorganic silicon material by plasma for covering the source region andthe drain region of the thin film transistor by an insulation filmenables further improvement in the reliability of the thin filmtransistor.

What is claimed is:
 1. A method of manufacturing a thin film transistor,said method comprising: forming a semiconductor thin film on atransparent substrate; forming a first insulation film having refractiveindex n1 and film thickness d1 on said semiconductor thin film; forminga gate electrode on said first insulation film; implanting dopant intosaid semiconductor thin film; forming a second insulation film havingrefractive index n2 and film thickness d2 in a way to cover said firstinsulation film and said gate electrode; and activating said implanteddopant by applying a laser with wavelength λ; wherein said filmthicknesses d1 and d2 are of sufficient thickness to cause said laser tobe reflected off of the gate electrode and to be absorbed by said thinfilm transistor away from said gate electrode, and wherein saidthickness d1 and d2 satisfy a set of the Formulae (1) and (2) when m andm1 are any given positive integers: d2*n2=2*m*λ/4  (1); andd1*n1+d2*n2=(2*m1−1)*λ/4  (2).
 2. The method of manufacturing a thinfilm transistor according to claim 1, wherein said first insulation filmis a gate insulation film.
 3. A method of manufacturing a thin filmtransistor, said method comprising: forming a semiconductor thin film ona transparent substrate; forming a first insulation film havingrefractive index n1 and film thickness d1 on said semiconductor thinfilm; forming a gate electrode on said first insulation film; implantingdopant into said semiconductor thin film; forming a second insulationfilm having refractive index n2 and film thickness d2 in a way to coversaid first insulation film and said gate electrode; and activating saidimplanted dopant by applying a laser with wavelength λ; wherein saidfilm thicknesses d1 and d2 are of sufficient thickness to cause saidlaser to be reflected off of the gate electrode and to be absorbed bysaid thin film transistor away from said gate electrode, and whereinsaid thickness d1 and d2 fall in a range satisfying a set of theFormulae (5) and (6) when m and m1 are any given positive integers:abs{d2*n2−2*m*λ/4}<λ/8  (5); andabs{(d2*n2+d1*n1)−(2*m1−1)*λ/4}<λ/8  (6).
 4. The method formanufacturing a thin film transistor as defined in claim 3, wherein therefractive index n2 of said second insulation film is substantiallyequal to the refractive index n1 of said first insulation film.
 5. Themethod for manufacturing a thin film transistor as defined in claim 1,wherein said first insulation film is silicon oxide made by decomposinga gaseous material at least containing organic silicon by plasma.
 6. Themethod for manufacturing a thin film transistor as defined in claim 3,wherein said first insulation film is silicon oxide made by decomposinga gaseous material at least containing organic silicon by plasma.
 7. Themethod for manufacturing a thin film transistor as defined in claim 1,wherein said semiconductor thin film is non-single crystal semiconductorthin film made of polycrystal silicon.
 8. The method for manufacturing athin film transistor as defined in claim 3, wherein said semiconductorthin film is non-single crystal semiconductor thin film made ofpolycrystal silicon.
 9. The method for manufacturing a thin filmtransistor as defined in claim 1, wherein dopant for forming source anddrain regions of said thin film transistor, and dopant for forming alightly doped drain (LDD) are implanted in said step of implantingdopant into said semiconductor thin film.
 10. The method formanufacturing a thin film transistor as defined in claim 3, whereindopant for forming source and drain regions of said thin filmtransistor, and dopant for forming a lightly doped drain (LDD) areimplanted in said step of implanting dopant into said semiconductor thinfilm.